Microelectronic circuits useful in electronic watches are typically formed as metal oxide semiconductor (MOS) devices on a single semiconductive substrate. The circuits are powered by a miniature battery and both the battery and the substrate are enclosed within the watch and control its display. The watch circuit typically includes a crystal oscillator which may be of the type disclosed in RCA COS/MOS Integrated Circuits Manual, RCA Solid State Division, Summerville, N.J., 1971, pages 138 through 148, the disclosure which is hereby incorporated herein by reference. A typical crystal oscillator complementary metal oxide semiconductor (CMOS) circuit includes a passive resonator such as a quartz crystal having two terminals connected across the input and output of an inverter amplifier comprising a P-channel MOS Field Effect Transistor (FET) and an N-channel MOSFET, the two MOSFETS having their drains connected together and to one terminal of the quartz crystal and having their gates connected together and to the other terminal of the quartz crystal. As discussed in the above-referenced RCA publication, the crystal oscillator will not function unless the oscillator loop gain is greater than unity. The disadvantage with this type of prior art oscillator is that, for a loop gain sufficiently greater than unity to assure reliable operation, the oscillator consumes a nominal amount of current on the order of one or two microamps. Because the miniature battery associated with the watch circuit has a limited capacity, the nominal current draw of the oscillator is a critical factor and must be minimized.
An improved crystal oscillator circuit of the prior art which has less current draw includes an N-channel FET having a two-terminal quartz crystal connected across its gate and drain. The current draw of the crystal oscillator is minimized by means of a P-channel FET connected between the N-channel FET drain and the power supply. A bias control circuit regulates the gate voltage of the P-channel FET in order to minimize the source-to-drain current through both the N and P-channel FET'S, significantly reducing the nominal current consumed by the crystal oscillator. However, the output from this circuit is too weak to drive additional elements in the watch circuit. Therefore, an additional amplifier stage is provided in order to boost the oscillator output so as to be useful in a watch circuit. Unfortunately, the additional amplifier stage consumes more current. Also, only the N-channel FET oscillates with the quartz crystal, the P-channel FET merely providing a regulated source of current. Therefore, for a given amount of oscillator gain, this improved crystal oscillator circuit requires at least approximately 1.6 times more current through the N-channel FET than the previously described crystal oscillator. Nevertheless, because of the presence of the amplifier stage, the gain requirement is less and, with current regulation, the improved crystal oscillator consumes less current than the previously described crystal oscillator. The limitations of the improved oscillator circuit are that, as oscillator stage current consumption is reduced, additional amplifier stage current is consumed. In addition only one FET oscillates with the crystal, thus requiring the one oscillating FET to draw more current to achieve a given amount of oscillator gain in comparison to a complimentary oscillator stage using both N-channel and P-channel MOSFETS. Thus, it has not seemed possible in the art to further significantly reduce the current consumption of crystal oscillators useful in watch circuits.